What's causing the unprecedented decline in numbers of honeybees? This mystery has been vexing scientists ever since U.S. beekeepers began noticing enormous numbers of their bees dying off or vanishing for no apparent reason several years ago.

Honeybees contribute some $15 billion to the U.S. economy every year, pollinating 90 major crops – everything from fruits to nuts. Most of us take these foods for granted, rarely realizing the vital role the tiny creatures play in making plants thrive.

Wikelski is pioneering the use of super-small radio tracking tags that fit on the backs of bees, a technological breakthrough that may provide him and other scientists with a direct view of the pollinators' flight patterns.

This could someday help them understand what's causing the honeybees'decline – and how to harness other kinds of bees to protect food supplies.

Until now, tracking insects such as bees has proved difficult. They fly too far too fast to be chased on the ground, and they're too small and nimble to monitor from the air. Bumblebees, for instance, cruise at 20 feet per second, says Rutgers University entomologist Rachael Winfree, who is working with Wikelski on the bee-tracking project. As a result, scientists are largely in the dark about basic questions involving bees' movements.

Most estimates of how far bees range when foraging are based on indirect tests such as removing bees a set distance from their nests to see if they return. “The assumption,” says Winfree, “is that the only way bees know where they are is because they have an internal map of the area, created when they flew there before. If displaced outside their mapped area, they can't navigate.”

But since some bee species are known to find their way back home from as far as 12 miles away, some scientists speculate the insects can also follow odor or magnetic gradients – or maybe they simply get lucky and find the nest as they buzz around aimlessly, Winfree says.

Wikelski, director of the Max Planck Institute of Ornithology in Seewiesen, Germany, has long pushed the frontiers of tracking small animals. He previously used radio tags to make breakthrough discoveries showing that songbird and dragonfly migration patterns are remarkably similar: They take advantage of favorable winds, take rest breaks and reorient themselves when they lose their bearings. He found one dragonfly that flew a whopping 100 miles in a single day.

In the bee-tracking project, Wikelski and his colleagues are using transmitters the size of three or four grains of rice, powered by a tiny hearing-aid battery and with a crystal-controlled oscillator and an antenna measuring up to an inch and a half. The transmitters, at a featherweight 0.006 ounces, are small and light enough to attach to the backs of bees from two relatively hefty species, weighing .02 ounces, with just a bit of eyelash glue and superglue.

Even loaded up with these backpacks, nearly a third of their body weight, “they fly beautifully,” says Wikelski.

The transmitters allow the scientists to track the insects as long as the bees remain within a few miles of their receiver. So far Wikelski and his team have fitted tags on orchid bees at Panama's Smithsonian Tropical Research Institute and conducted successful indoor tests in a New Jersey lab with North America's biggest bee species, the carpenter bee.

These early tests are proof of concept. Most bees are much smaller than orchid and carpenter bees. In fact, many wild bee species are the size of just a pine nut. Winfree says the team tried to fit transmitters on bumblebees, but these .007-ounce insects simply couldn't carry the load, which amounted to about 80 percent of their body weight.

“You would throw them up into the air, and they would just come back to the ground,” she says. “So we need a next generation of transmitters for them, I think.”

For now, Wikelski says, the radio tags for the carpenter bees are as small as current technology allows. But he thinks they can be made even smaller and hopes his engineering colleague James Cochran, who made the 170-milligram tags, will be able to shrink them by another 40 percent or so – down to just 100 milligrams.

Smaller tracking tags like this may eventually help scientists address growing concerns about the future production of crops such as apples, melons, and almonds that require bees for pollination. Even though domesticated honeybee colonies, which currently pollinate most U.S. crops, are mysteriously collapsing, native wild bees appear unaffected by the so-called colony collapse disorder.

Studies by Winfree and Claire Kremen, a conservation biologist at the University of California Berkeley, have shown that in some cases, native bees alone can fully pollinate crops. This means there's reason for hope if the honeybee species vanish, though native bees could never completely replace domesticated ones in most of the farming operations that currently depend on them.

One key will be to maintain native bee habitats within range of crops needing the bees' services. And that requires understanding how far bees will fly to feed on the crops, notes Winfree. “Putting a transmitter on these species and seeing where they go and how far is obviously the way to nail that.”

Tiny tracking tags should eventually enable scientists to tackle a laundry list of other questions, including how bees and other pollinators interact with flowering plants in rain forests and how to design agricultural landscapes to attract and sustain native bees.

Tracking studies may eventually help confirm a hypothesis that bees, like most animals, move only as far as the nearest food source – even if they have the ability to fly farther.

Scott Hoffman Black, executive director of the Xerces Society for Invertebrate Conservation, an organization based in Portland, Ore., that advocates for the protection of pollinators, says tagging bees in food-rich and food-poor landscapes may lead to better, more efficient uses of farmland.

For now, all these possibilities remain to be explored. But, says Wikelski, “We're at the verge of something very exciting.”